Let me guess - the sunroof clutch is designed to prevent the motor from pulling too hard on the cable when it either closes or opens all the way.

Therefore, when the cable snaps or the motor burns out... then the clutch is too tight.

If it was me, I'd treat it like a 2-man operation....... have an assistant work the sunroof, keeping the electric switch pressed beyond closing and opening the roof - while you climb into the trunk with a flashlight, close the trunk lid and study how the motor behaves.

The purpose of locking yourself in the trunk is so that you will have utter silence for studying the pitch of the electric motor.